Integrated System for Ultrasound Imaging and Therapy using Per-Pixel Switches

ABSTRACT

Ultrasound imaging and therapy with the same array of capacitive micromachined ultrasonic transducers is provided. The electronics includes a per-pixel switch for each transducer element. The switches provide an imaging mode driven completely by on-chip electronics and a therapy mode where off-chip pulsers provide relatively high voltages to the transducer elements.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional patentapplication 62/021,341, filed on Jul. 7, 2014, and hereby incorporatedby reference in its entirety.

GOVERNMENT SPONSORSHIP

This invention was made with Government support under contract numberR01HL117740-01A1 awarded by the National Institutes of Health. TheGovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to combined ultrasound imaging and therapy.

BACKGROUND

Ultrasound is used in medicine for both imaging and therapy. However,conventional ultrasound imaging and therapy approaches tend to requiredifferent systems for imaging and therapy. This can undesirably lead toincreased time in diagnosis and treatment of the patient. Also, aphysician may need to use a “best guess” strategy fortherapy/stimulation of a particular region because of lack of real-timespatial and anatomical information of the region of interest. Therefore,methods of providing both imaging and therapy with the same system havebeen considered.

One example is considered in US 2009/0240148. In this work, the systemincludes both an imaging array and a therapy array integrated to form acomposite array. Thus some parts of the array are dedicated to imagingand other parts of the array are dedicated to therapy.

Another example is considered in US 2014/0288428. In this work, thesystem includes a single transducer array used for both imaging andtherapy, and all corresponding electronics is monolithically integratedwith the transducer array.

SUMMARY

We have found that known approaches for providing both imaging andtherapy in a single ultrasound system have undesirable drawbacks. Incases where a composite imaging array is used, extra complexity may beincurred by having some parts of the array dedicated to imaging andother parts of the array dedicated to therapy. In cases where fullmonolithic integration is employed, the on-chip power dissipation causedby the relatively high-voltage pulsers used for ultrasound therapy canbe excessive.

This work alleviates the above-described drawbacks by providing atransducer configuration where the electronics includes a per-pixelswitch for each transducer element. The switches provide an imaging modedriven completely by on-chip electronics and a therapy mode whereoff-chip pulsers provide relatively high voltages to the transducerelements.

Furthermore, it is important that the transducer elements be capacitivemicromachined ultrasonic transducer (CMUTs), as opposed to piezoelectrictransducers. One reason for this is that CMUTs dissipate far less heatin the therapy mode than piezoelectric transducer do. Another importantfactor is the surprising ability of a single CMUT design to work wellfor both imaging and therapy.

Some design considerations relating to CMUT design for both imaging andtherapy follow. Ultrasound imaging has typically used transducers thatoperate in frequency ranges from 5-15 MHz. Two-dimensional CMUT arraysoperating at 5 MHz have been developed to achieve volumetric imaging.The wide bandwidth nature of the transducer (low Q) provides superiorimaging qualities due to better image resolution, when compared toconventional piezoelectric transducers.

However, using the same transducer for therapy has its trade-off—onebeing inferior penetration performance (typical transducers used fortherapeutic application operate at 1 MHz) and the other being the factthat these transducers have a low mechanical Q (high Q devices aredesirable for therapeutic application). One way to compensate for thelow Q is to use series inductors and tune them to achieve as high a Qpossible. The benefit of doing this is the reduced voltage driverequirements of the pulsers. The drive voltage seen by the transducer isQ times amplified allowing for reduced power dissipation by the drivingcircuit. Such a scheme can make dual-modality of the CMUT system morepractical, where, in one mode, we use the wide bandwidth nature of CMUTsto achieve high resolution imaging, and in the other mode, we use theinductors to tune the front end transducers to achieve high Q, allowingfor a better therapeutic system.

This approach has widespread applications. Any ultrasound applicationthat requires simultaneous imaging and HIFU(high intensity focusedultrasound)/therapy capabilities can benefit. For example, applicationsinclude ultrasound imaging and neural stimulation.

Significant advantages are provided. At present, ultrasound imaging andtherapy/stimulation usually needs to be done using different devicesleading to increased time in diagnosis and treatment of the patient.Also, physicians may need to use a “best guess” strategy to determinethe treatment location since it does not have real-time spatial andanatomical information of the region of interest. Use of the presentapproach will allow one to remove such uncertainties in the actualregion of interest and improve the speed of diagnosis/treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment of the invention.

FIG. 2 shows a 2-D array of transducer elements.

FIG. 3 shows one of the transducer elements of the array of FIG. 2.

FIG. 4 shows circuitry to perform ultrasonic therapy with the 2-D arrayof transducer elements.

FIG. 5 is a more detailed view of per-pixel circuitry.

FIG. 6 shows a preferred circuit for mode switching.

FIG. 7 shows a pixel switch configured to connect or disconnect itscorresponding transducer element to a selectable one of the off-chippulsers.

FIG. 8 shows another embodiment of the invention.

FIG. 9 shows a further embodiment of the invention.

DETAILED DESCRIPTION

This work provides a novel method for integrating ultrasound imaging andtherapy (or ultrasound neuromodulation) using the same ultrasoundtransducer array integrated with electronics. Having the same device forboth purposes can be very beneficial. However, there are challenges inincorporating imaging and continuous-wave (CW) or quasi-CW ultrasoundapplication capabilities in a single integrated chip (such as powerdissipation, and area requirements). This work provides a solution tothese challenges using switches in the integrated circuit that canswitch between an imaging mode (Mode I) and a CW/quasi-CW mode (ModeII). The switches enable the use of external off-chip pulsers for ModeII allowing it to outsource the power dissipation to the back-endsystem.

A 2D CMUT array can be integrated with an IC (integrated circuit) thatincludes transceivers for using the integrated chip for imaging. Thetransceivers include transmit beam-formers that are capable of providinga high voltage (HV) pulse and receivers that signal condition thereceive signals from the CMUT array. For Mode I, a single (or veryshort) HV pulse is used (duty cycle of the pulse is miniscule). However,for Mode II, multiple cycles of HV pulsing is desirable, to increase thetime-averaged intensity of the focal spot. This leads to large powerdissipation of the IC chip, if on-chip pulsers are used, since there aremultiple elements pulsing. Therefore, using on-chip pulsers for Mode IIapplications is impractical.

This problem is alleviated by using HV switch(s) in each element, thatcan route the connection of the ultrasound transducer array elementdirectly to an off-chip HV pulser. Though the overall power dissipationstill remains high, the dissipation occurs at the back-end which is notnear the patient. Utilizing these switches allow us to switch a singlesensor array between Mode I and Mode II. Different elements can bepulsed at different phases to provide focusing in Mode II. In oneimplementation, we use 8 off-chip pulsers with equally spaced phases anddistribute the elements amongst these 8 pulsers to enable focusing at agiven spot. With such a configuration, we can use the same integratedarray for Mode I as well as for Mode II, as and when desired by thephysician when performing a medical procedure.

FIG. 1 shows an exemplary embodiment of the invention. In this example,a 2-D CMUT array 102 is flip chip bonded (112) to chip 104 that includesimaging electronics and the per-pixel switches. Off-chip pulsers 108provide a high-voltage assembly 106 that is connected to chip 104 viabus 110. Bus 110 has one line for each pulser 108.

FIG. 2 is a plan view of the 2-D array of transducer elements 102. Oneof the transducer elements is referenced as 202. FIG. 2 shows a 6×6array of transducers, but any other 2D array dimensions can also beused. The low on-chip power dissipation of the present approach canenable large array sizes (e.g., 32×32 or larger). FIG. 3 showstransducer element 202 in more detail. As indicated above, thetransducer elements are CMUTs. The general configuration for a CMUT isshown here, with 306 being an elastic membrane suspended above asubstrate 302 by one or more support members 304.

FIG. 4 shows circuitry to perform ultrasonic therapy with the 2-D arrayof transducer elements. Here chip 104 is regarded as having cells thatcorrespond to the elements of transducer array 102. Bus 110 is shownbeing available at all cells, and one of these cells is referenced as402. FIG. 5 is a more detailed view of cell 402. Here 502 is a flip chippad that is connected to the corresponding transducer element oftransducer array 102, 504 is the per-pixel switch, 506 is on-chipimaging circuitry, and 508 is an optional inductance in series with theCMUT capacitance that is present in preferred embodiments to provideelectrical resonance in the therapy mode.

Thus an exemplary embodiment of the invention is apparatus forultrasonic imaging and therapy, where the apparatus includes:

1) a 2-D array of transducer elements monolithically integrated on atransducer chip (102), wherein each of the transducer elements is acapacitive micromachined ultrasonic transducer (FIG. 3);

2) first electronic circuitry (506) configured to perform ultrasonicimaging with the 2-D array of transducer elements, wherein the firstelectronic circuitry is integrated with the transducer chip (e.g.,hybrid integration as on FIGS. 1 and 8, or monolithic integration as onFIG. 9);

3) second electronic circuitry (106) configured to perform ultrasonictherapy with the 2-D array of transducer elements, wherein the secondelectronic circuitry comprises one or more off-chip pulsers (108)disposed remotely from the transducer chip; and

4) mode switching circuitry (504) configured to switch operation of eachof the transducer elements between an ultrasonic imaging mode and anultrasonic therapy mode.

As indicated above, it is preferred that the second electronic circuitryinclude an inductor corresponding to each transducer element configuredsuch that the combination of inductor and transducer element iselectrically resonant at the ultrasound therapy frequency. Thiselectrical resonance helps to compensate for the relatively lowmechanical Q of the CMUT transducer elements. There are several optionsfor the location of the series inductors. The inductors can be locatedinside block 504, e.g., 508 on FIG. 5. Another option is to have anotherdie with just inductor arrays.

FIG. 6 shows a preferred circuit 504 for mode switching, without theoptional inductor 508. Here 602 is the control input to the switch, 604is an input from the corresponding off-chip pulser, and 606 is aconnection to the corresponding CMUT transducer element. In the circuitof FIG. 6, devices D2, Q3, Q1 and Q2 need to be high voltage devices(i.e., capable of handling the voltages provided at input 604 by thepulser), while devices D1 and Q4 can be low voltage devices. The neteffect of this circuit configuration is to allow a drive voltage to beapplied to transistors Q1 and Q2 that is greater than the gate-sourcevoltage of any single transistor in the circuit. This configuration canbe regarded as a first pair of series-connected transistors and a diodehaving an output that drives a second pair of series-connectedtransistors and a reverse-biased diode.

The off-chip pulsers 108 can be configured to provide inputs havingdistinct electrical phase. This can provide beam control for ultrasonictherapy. In some cases each pixel switch is configured to connect ordisconnect its corresponding transducer element to a predetermined oneof the off-chip pulsers. FIG. 5 shows an example of this configuration.The resulting beam forming for therapy is fixed (e.g., focusing in thecenter of the imaging field of view). In other cases each pixel switchis configured to connect or disconnect its corresponding transducerelement to a selectable one of the off-chip pulsers. FIG. 7 shows anexample of this configuration, where line switch 702 selects the line ofbus 110 that is connected to block 504.

The resulting beam forming for therapy is adjustable. For example, thetherapy beam can be brought to a focus at any selected location withinthe imaging field of view. As another example, the therapy beam could beconfigured to scan within the imaging field of view along any desiredpath.

Practice of the invention does not depend critically on details of howintegration is performed, provided that the pulsers for the therapy modeare not integrated with the transducer chip. FIG. 1 shows a two-chiphybrid bonded configuration. In the specific example given above, boththe first electronic circuitry (for imaging) and the mode controlcircuitry (for switching) were included in chip 104. Other two-chipconfigurations can be obtained by moving the imaging or switchingcircuitry to be monolithically integrated with transducer chip 102. Inthese cases, chip 104 will have only the switching circuitry or only theimaging circuitry, respectively.

Three-chip configurations are also possible. FIG. 8 shows an example,where a switch chip 802 and an imaging chip 804 are integrated withtransducer array 102. Here it is assumed that mode switching circuitry808 is on the top side of switch chip 802, and that through-chip vias810 are used to connect mode switching circuitry 808 to imaging chip804. Imaging chip 804 is flip chip bonded (806) to switch chip 802.

A final possibility is full monolithic integration of everything exceptthe off-chip pulsers. FIG. 9 shows a example of this approach. Here thefirst electronic circuitry (for imaging), the second electroniccircuitry (except for pulsers 108) and the mode switching circuitry areall monolithically integrated on transducer chip 902.

1. Apparatus for ultrasonic imaging and therapy, the apparatuscomprising: a 2-D array of transducer elements monolithically integratedon a transducer chip, wherein each of the transducer elements is acapacitive micromachined ultrasonic transducer (CMUT); first electroniccircuitry configured to perform ultrasonic imaging with the 2-D array oftransducer elements, wherein the first electronic circuitry isintegrated with the transducer chip; second electronic circuitryconfigured to perform ultrasonic therapy with the 2-D array oftransducer elements, wherein the second electronic circuitry comprisesone or more off-chip pulsers disposed remotely from the transducer chip;mode switching circuitry configured to switch operation of each of thetransducer elements between an ultrasonic imaging mode and an ultrasonictherapy mode.
 2. The apparatus of claim 1, wherein the mode switchingcircuitry comprises a pixel switch corresponding to each of thetransducer elements, wherein each pixel switch is configured to connector disconnect its corresponding transducer element to a predeterminedone of the off-chip pulsers.
 3. The apparatus of claim 1, wherein themode switching circuitry comprises one or more pixel switchescorresponding to each of the transducer elements, wherein each one ormore pixel switches is configured to connect or disconnect itscorresponding transducer element to a selectable one of the off-chippulsers.
 4. The apparatus of claim 1, wherein the first electroniccircuitry is an integrated circuit on a first electronics chip andwherein the first electronics chip is hybrid integrated with thetransducer chip.
 5. The apparatus of claim 4, wherein the mode switchingcircuitry is an integrated circuit on a second electronics chip andwherein the second electronics chip is hybrid integrated with thetransducer chip.
 6. The apparatus of claim 1, wherein the mode switchingcircuitry is an integrated circuit on a second electronics chip andwherein the second electronics chip is hybrid integrated with thetransducer chip.
 7. The apparatus of claim 1, wherein the firstelectronic circuitry, the second electronic circuitry and the modeswitching circuitry are all monolithically integrated on the transducerchip.
 8. The apparatus of claim 1, wherein the off-chip pulsers areconfigured to provide inputs having distinct electrical phase, wherebybeam control for ultrasonic therapy is provided.
 9. The apparatus ofclaim 1, wherein the mode switching circuitry comprises at least oneswitching circuit configured as a first pair of series-connectedtransistors having an output that drives a second pair ofseries-connected transistors.
 10. The apparatus of claim 1, wherein thesecond electronic circuitry includes an inductor corresponding to eachtransducer element configured such that the combination of inductor andtransducer element is electrically resonant at an ultrasound therapyfrequency.